Origin of intracameral sheets in ammonoids

The distribution, morphology and mutual relationships of cameral sheets in ammonoids are revised and re-evaluated. Taking into account recent models of ammonoid septum and chamber formation, three different origins can be attributed to the morphological types of sheets: (1) membranes replicated by the rear mantle (pseudosepta and septal linings), (2) membranes secreted sequentially and/or stretched across the chamber (horizontal membranes and chamber linings) and (3) products of desiccation of the cameral liquid (transverse and siphuncular sheets), presumably a cameral hydrogel. Sheets are always preserved near the siphuncular area, because as the cameral liquid was pumped out from the chamber it became progressively richer in dissolved mucus. In the last-formed drops, or menisci, this mucus adhered to the surface of the previously secreted sheets and, on dehydration, it also replicated the surface of the residual reservoirs, producing desiccation sheets. On the basis of the new evidence, changes in the shape of the rear mantle in Triassic ammonoids can be reconstructed. In general, deformations affected the rounded or bottle-neck saddles, which deflated after detachment of the last-formed septum and reinflated when the position of the next septum was reached. The rest of the elements of the septal epithelium were affected to a much lesser extent. One of the functions of those cameral sheets secreted by the rear body was related to a more efficient transport of the cameral liquid upon decoupling from the siphuncular tube. Ammonoids, Triassic, septum, chamber growth, cameral sheets.